White light is passed through a Michelson interferometer in one of whose arms a thin lamina of the substance in the shape of a tension member is supported by a loading frame. The emergent beam, examined by a spectrometer, shows a continuous spectrum crossed by interference fringes. For the unstressed lamina the position of the fringes gives the variation of refractive index with wavelength For the stressed lamina the behaviour of the fringes gives the stress optical coefficients. No modification of the interferometer, and only three sets of observations on the fringes, are required. For the calculation of absolute values of the refractive index the values of two wavelengths must be known.Full details of the method are given, and photographs of tests on Columbia Resin 39 are included. These show that CR39 becomes positively birefringent under tension